Shape memory alloys and superelastic alloys exhibit a remarkable shape memory effect and superelastic characteristics concomitantly to reverse transformation of the thermoelastic martensite transformation, and have excellent functions near the living environment temperature. Accordingly, these alloys have been put to practical use in various fields. Representative alloys of the shape memory alloys and superelastic alloys include TiNi alloys and copper (Cu)-based alloys. Copper-based shape memory alloys and superelastic alloys (hereinafter, those are simply referred to copper-based alloys) have characteristics inferior to those of TiNi alloys in terms of repetition characteristics, corrosion resistance, and the like. However, since the cost is inexpensive, there is a movement to extend the application range of copper-based alloys.
As an example of putting the copper-based alloy to practical use, the inventors of the present invention have proposed a correcting tool for ingrown toenail (so-called ingrown nail), which is comprised of a material obtained by subjecting to a shape-memory heat treatment on a cold-rolled sheet of a Cu—Al—Mn-based superelastic alloy, and which is excellent in durability of a nail-holding portion to repeated stress application (Patent Literature 1). The correcting tool for ingrown toenail described in Patent Literature 1 is intended to provide a correcting tool for ingrown toenail that is excellent in durability of a mounting portion (the nail-holding portion), in consideration of the use under the environment in which repeated deformation stress is applied. Further, in Patent Literature 1, regarding stress corrosion, prevention of occurrence of stress corrosion cracking in a stress corrosion test using an artificial perspiration (sweat) is tested and evaluated, in consideration of the case where a user goes about his/her daily life in a state where the correcting tool for ingrown toenail is mounted on the ingrown toenail.
Further, there is proposed austenitic stainless steel that is not a shape memory alloy or a superelastic alloy but that is intended to prevent occurrence of stress corrosion cracking (hereinafter, also referred to as SCC) by controlling a low sigma coincidence grain boundary frequency (Patent Literature 2).